Telescopes

Introduction

• A telescope is designed to form on the retina of the eye a larger image of an object than would be created if the object were viewed with the unaided eye.
• In this experiment three different types of telescopes will be constructed.
• First let’s look at different lens shapes.

Types of Lenses

Double

Convex

Double

Concave

Concave Meniscus

Plano

Convex

Plano

Concave

Convex Meniscus

Farsighted people use lenses similar to these.

Nearsighted people use lenses similar to these.

Types of Lenses

Double

Convex

Double

Concave

Concave Meniscus

Plano

Convex

Plano

Concave

Convex Meniscus

Convex lenses are thicker in the center

than they are at the edges.

Types of Lenses

Double

Convex

Double

Concave

Concave Meniscus

Plano

Convex

Plano

Concave

Convex Meniscus

Convex lenses converge light rays; concave lenses diverge light rays.

The three telescopes are:

• Astronomical
• Terrestrial
• Galilean (opera glass)

To understand how they operate one should first see how an image is created by a single lens.

A Convex Lens Converges Light Rays

A Concave Lens Diverges Light Rays

Imaging with a Convex Lens

sees an

image here.

Convex Lens

Arrow as

Object

A ray parallel to the principal axis

is bent upon entering the lens.

Upon exiting the lens it is bent again

and passes through a point

called the focal point.

A ray passing through the center of the lens is basically undeflected.

This arrangement produces an inverted, real, diminished image. Image descriptions follow.

Image Descriptions

• Inverted image means that the image is up-side-down compared to the object.
• Real image means that the image can be viewed by the reflection from a screen placed at the image’s location. The light rays forming the image actually pass through the physical location of the image.

• Diminished image means that it is reduced in size compared to the size of the object.

More Imaging �With a Convex Lens

sees an

image here.

Convex Lens

Arrow as

Object

A ray parallel to the principal axis

is bent upon entering the lens.

Upon exiting the lens it is bent again

and passes through a point

called the focal point.

A ray passing through the center of the lens is basically undeflected.

This arrangement produces an upright, virtual, magnified image. It is a simple magnifying glass.

Image Descriptions

• Upright image means that the image is in the same up or down orientation as the object.
• Virtual image means that the image cannot be formed on a screen. The light rays forming the image only appear to pass through the physical location of the image.
• Magnified or enlarged image means that it is larger in size compared to the size of the object.

Imaging with a Concave Lens

sees an

image here.

Concave Lens

Arrow as

Object

A ray parallel to the principal axis

such that is appears to have come

from a point called the focal point.

is bent upon entering the lens.

Upon exiting the lens it is bent again

A ray passing through the center of the lens is basically undeflected.

This arrangement produces an upright, virtual, diminished image

no matter where the object is located.

Astronomical Telescope

• The astronomical telescope is composed of an objective lens (convex) and an eyepiece lens (convex).
• As demonstrated in an earlier slide the objective lens by itself produces a diminished, real, inverted image of a distant object being viewed. (Note: The final image produced by the astronomical telescope will be magnified, virtual, inverted.)
• For distant objects the image (produced by the objective lens) is just outside the focal point of the objective lens.

The Objective Lens of an Astronomical Telescope

Objective (Convex Lens)

The rays from a distant object (arrow)

This real image becomes the

object for the eyepiece.

Eyepiece of an�Astronomical Telescope

• The eyepiece lens is used as a simple magnifying glass to observe the image formed by the objective lens.
• This image formed by the eyepiece is a virtual image of the real image produced by the objective.
• The observer will see an inverted image with this type of telescope.

The Eyepiece Lens of an Astronomical Telescope

Eyepiece

(Convex Lens)

The image formed by the objective

becomes the object for the eyepiece.

The Astronomical Telescope

f

Eyepiece

(Convex Lens)

(Shorter focal length)

Objective

(Convex Lens)

(Longer focal length)

The rays from

a distant

object (arrow)

This real image becomes the

object for the eyepiece.

Inverter Lens in a�Terrestrial Telescope

• A third lens can be introduced between the objective lens and the eyepiece lens of an astronomical telescope in such a way as to produce an upright image for the observer.
• This lens effectively transforms the astronomical telescope into a terrestrial telescope.

​

The Inverter Lens in a�Terrestrial Telescope

Inverter

(Convex Lens)

The image formed by the objective

becomes the object for the inverter.

The inverter simply inverts the image.

The Terrestrial Telescope

f

f

The rays from

a distant

object (arrow)

f

f

Eyepiece

(Convex Lens)

(Shorter focal length)

Objective

(Convex Lens)

(Longer focal length)

Inverter

(Convex Lens)

(Shorter focal length)

Eyepiece

(Convex Lens)

(Shorter focal length)

Objective

(Convex Lens)

(Longer focal length)

The rays from

a distant

object (arrow)

Inverter

(Convex Lens)

(Shorter focal length)

The Terrestrial Telescope

Galilean Telescope�or Opera Glass

• The objective lens is convex and the eyepiece lens is concave.
• The resultant image is upright, magnified, and virtual.
• The field of view is quite limited, which is okay for operas but is not very useful for watching football games for example.

The Eyepiece lens of a �Galilean Telescope (Opera Glass)

Eyepiece

(Concave Lens)

Light from the objective

would see an image here.

The Galilean Telescope (Opera Glass)

Eyepiece

(Concave Lens)

(Shorter focal length)

Objective

(Convex Lens)

(Longer focal length)

Magnification of a Telescope

Example

Telescope Summary Follows

The Astronomical Telescope

Eyepiece

(Convex Lens)

(Shorter focal length)

Objective

(Convex Lens)

(Longer focal length)

The Terrestrial Telescope

Eyepiece

(Convex Lens)

(Shorter focal length)

Objective

(Convex Lens)

(Longer focal length)

Inverter

(Convex Lens)

(Shorter focal length)

The Galilean Telescope�(Opera Glass)

Eyepiece

(Concave Lens)

(Shorter focal length)

Objective

(Convex Lens)

(Longer focal length)